1. Field of the Invention
This invention relates to a method of making a metal-coated fiber, and fibers made thereby. In one embodiment, the fiber to be coated is a silica glass optical fiber having a polymer inner coating layer.
2. Description of the Prior Art
Optical fibers for communications and other uses are typically made of silica glass or plastic, but can be made of other materials. Dopants such as germanium, phosphorus, boron, fluorine, etc., are typically included to obtain a desired index of refraction profile in the fiber, or to facilitate manufacture of the fiber, or for other purposes. Whatever the material, it is usually desirable to protect the fiber from abrasion, water entry, and microbending losses, among other things. For this purpose, at least one coating is typically applied to an optical fiber during manufacture. Most typically, a coating layer is applied during the drawing of the fiber from a heated preform comprising glass or other material of which the optical fiber is made. This is referred to as an in-line process. However, a fiber can alternately be formed in a first operation and later coated in a separate operation. At present, organic coatings, typically polymers, are applied in-line using open cup applicators. The extrusion of nylon or other polymers onto a fiber is also known in the art.
Concurrent with the polymer coating technology, several techniques have been developed for applying inorganic materials to fibers. In certain cases, inorganic materials, for example metals, are superior to polymer coatings. This is especially true when prevention of moisture entry into contact with the fiber is desired. Moisture entry can reduce the tensile strength of the fiber and produce other undesirable effects. This is especially significant, for example, in fiber designed to be used under the ocean or to be directly buried in the ground, wherein outer layers of cabling cannot be relied upon to entirely prevent moisture entry over long periods of time in some cases. Furthermore, a metal coating on a fiber can act as a conductor for transmission of electrical power for a repeater, or for signaling, or to ensure against unauthorized access to the fiber, among other purposes.
Unfortunately, metal coatings applied directly to a glass optical fiber can in some cases degrade it through chemical action and slip plane intersection. The latter mechanism produces hardening centers at the glass-metal interface which are thought to increase microbending losses. Therefore, if a metal coating is to be applied, it should be deposited over an organic undercoating. However, the method of applying the metal coating to the organic layer must result in a sufficiently low temperature so that substantial degradation of the organic layer is avoided. This is also the case wherein the fiber itself comprises polymer material, as in the case of plastic fibers.
Typical present-day methods of applying metals to fibers include vapor deposition, plasma, jets, or freeze coatings. Another method of coating an optical fiber with a metal is to pass a silica fiber through a molten bead of a metal; for example, aluminum or an aluminum alloy. This allows for relatively high speed coating of the fiber. Unfortunately, the molten metal method requires a temperature too high for application to many polymer materials without degradation. The prior art metal application methods also typically suffer from a virtual absence of wetting between the metal coating and an organic material, which makes application difficult to polymers and other organic materials. Some metals are difficult to apply to silica glass or other inorganic materials. Therefore, it is desirable to find an alternate method of applying a metallic layer onto an optical fiber.